This invention relates to apparatus and methods for measuring noise in a device. In particular, this invention relates to apparatus and methods for measuring noise in a transistor device.
Circuit designers typically use design tools to design integrated circuits. The most common design tools are the so-called simulated-program-with-integrated-circuit-emphasis (SPICE) and the fast device level simulators (e.g., Star-Sim, ATS, MACH TA, and TIMEMILL). Typically, design tools, such as SPICE and fast device level simulators, describe individual device and its connections in a line-by-line manner. Examples of individual devices are resistor, capacitor, inductor, bipolar junction transistor, and metal oxide semiconductor field effect transistor (MOSFET). In a design tool, each line, which includes a description of a device, is sometimes referred to as a device specification instance.
FIG. 1A illustrates an exemplary netlist developed by a design tool, such as SPICE. As shown in FIG. 1A, a netlist 101 typically includes three sections: a circuit description section 103, a models section 105, and an analysis section 107. The circuit description section 103 contains a description of each device and sub-circuit as well as interconnections between the devices and sub-circuits within an integrated circuit. The models section 105 contains a description of individual device and sub-circuit behavior. Typically, the models section 105 comprises a library of model parameters, model parameter values, and model equations. Generally, the behavior of each type of device (e.g., a MOSFET) can be simulated by at least one model equation, which includes a combination of model parameters. The analysis section 107 typically includes analysis instructions to simulate a device, sub-circuit, or circuit (e.g., output voltage over time) using information in the circuit description section 103 and the models section 105.
In order to meet continuous demands to reduce manufacturing costs and device size, many manufacturers have begun to manufacture chips that contain all components of an integrated circuit. Although less expensive to manufacture, such chips are more expensive to design. One important design factor is to measure and account for device and chip noise. Noise is a phenomenon that exists in analog or digital devices in an integrated circuit. Typically, noise is inversely proportional to device size. That is, as device size decreases, noise increases. Thus, as the industry continues to reduce device size, it is increasingly important to be able to measure and account for noise during integrated circuit design.
In most existing noise measurement systems, three components are present: a biasing system for biasing a device under test (DUT), a noise amplifier for amplifying noise measured from the DUT, and a control unit for controlling the biasing system and the amplifier. An efficient noise measuring system requires that the noise measuring system itself does not generate noise that interferes with or engulfs the noise measured from the DUT.
In some existing biasing systems, batteries are used to bias a DUT. Batteries are pure chemical power sources, thus, are relatively noiseless. One drawback of using batteries as a power source is that the voltage provided by a battery is typically not adjustable. Some biasing systems use a potential meter to adjust battery voltage. However, a potential meter generally produces unwanted thermal noise when used. In other existing biasing systems, an electronic circuit is connected to a battery to adjust battery voltage. Using such electronic circuit, for example a digital-to-analog converter, is preferable over using a potential meter because the electronic circuit is generally quieter. However, designing an effective and quiet electronic circuit can be very expensive; thus, this solution is not widely adopted. In yet other existing biasing systems, a commercial programmable DC power supply is used. Such commercial DC power supply can be programmed to provide adjustable biasing voltages. A drawback of the commercial DC power supply is that the power supply itself generates noise.
In most noise measurement systems, a noise amplifier is used to amplify a weak DUT noise to a measurable level. To be effective, the amplifier should generate minimal noise and that amplifier noise should be distinguishable from the measured DUT noise. Further, ideally, the noise amplifier should be capable of amplifying a wide range of noise frequencies (e.g., between 0 Hz and 1 MHz). Generally, an amplifier has its own input impedance. Amplifier performance generally improves if the amplifier impedance is closely matched by the impedance of the DUT. In most existing amplifiers, the amplifier impedance is generally fixed or very difficult to adjust. Thus, to achieve impedance matching, the DUT impedance should be adjustable to match the amplifier impedance.
Thus, it is desirable to provide apparatus and methods for effectively measuring noise in a device such as a transistor device.
An exemplary apparatus for measuring noise in a device comprises a plurality of programmable power supply units, a plurality of filter circuits coupled to the power supply units and selective terminals of a device, a variable loading resistor circuit coupled to a first terminal of the device, a calibration circuit coupled to a second terminal of the device, an amplifier circuit coupled to the first terminal of the device, and an output analyzer coupled to the amplifier circuit. The calibration circuit calibrates a gain of both the device and the amplifier circuit under each bias condition. In one embodiment, each of the plurality of filter circuits comprises a variable resistor and a capacitor coupled to the variable resistor. In another embodiment, the variable loading resistor circuit comprises a plurality of resistors selectably coupled in parallel, such that each resistor of the plurality of resistors can be turned on via a switch individually or in combination with other resistors of the plurality of resistors. In yet another embodiment, the variable loading resistor circuit further comprises a switch for shorting the variable loading resistor circuit.
In an exemplary embodiment of the apparatus, the amplifier circuit comprises a first phase amplifier circuit and a second phase amplifier circuit. In one embodiment, the first phase amplifier circuit includes a switch assembly. The switch assembly switchable coupling a voltage amplifier, a current amplifier, or a short circuit based on the impedance of the device. In another embodiment, the first phase amplifier circuit is configured to operate in a voltage amplifier mode or a current amplifier mode based on the impedance of the device. The second phase amplifier circuit further amplifies signals received from the first phase amplifier circuit.
In another exemplary embodiment, the apparatus further includes a protection circuit for discharging accumulated charge. The protection circuit is switchable coupled to the second terminal of the device. In yet another exemplary embodiment, the apparatus further includes a variable input resistor that is switchable coupled between the second terminal of the device and a filter circuit. In one embodiment, the variable input resistor is connected when the device is a bipolar transistor or a deep submicron device.
An exemplary method for measuring noise in a device comprises receiving a bias condition, measuring direct current operating points based on said bias condition, determining a loading resistor value based on a device impedance and an amplifier impedance, calculating a supply voltage based on the direct current operating points and the loading resistor value, selecting an amplifier mode based on the device impedance, calibrating a device gain and an amplifier gain, measuring noise data under the bias condition, removing an undesired portion of the noise data to obtain device noise data, extracting a model based on the device noise data, and simulating the extracted model. In one embodiment, calculating a supply voltage includes calculating an initial voltage value, measuring a supply current, comparing the supply current to a sum of a leak current and a device current, setting the supply voltage to the initial voltage if the supply current is equal to the sum, incrementing the supply voltage by a change voltage if the supply current is not equal to the sum and repeating the measuring and comparing. In another embodiment, calculating a supply voltage further comprises receiving an error percentage and calculating the change voltage based on the error percentage.
In an exemplary embodiment, an amplifier mode is selected by measuring noise densities at a plurality of loading resistance and selecting a voltage amplifier mode or a current amplifier mode based on the measuring. In another exemplary embodiment, a voltage amplifier mode is selected if the device impedance is low and a current amplifier mode is selected if the device impedance is high. In another exemplary embodiment, device and amplifier gains are calibrated by providing an alternate current signal at an input terminal of the device, measuring an output voltage at an output terminal of the device, and calculating the device gain and the amplifier gain based on the input voltage and the output voltage.
In one embodiment, an undesired portion of the noise data is removed in a de-embedding process that includes determining a system noise and separating the system noise from the noise data. In an exemplary embodiment, extracting a model includes extracting a direct current model having direct current model parameters, extracting frequency dependent model parameters, extracting noise data at one or more sampling frequency, associating the noise data to at least one bias condition, and extracting a noise model based on the associating and the direct current model parameters. In another exemplary embodiment, simulating the model includes generating a test circuit based on the model and simulating the test circuit.
An exemplary computer program product for measuring noise in a device comprises logic code for receiving a bias condition, which indicates the condition to measure noise in the device, logic code for measuring direct current operating points based on the bias condition, logic code for determining a loading resistor value based on a device impedance and an amplifier impedance, logic code for calculating a supply voltage based on the direct current operating points and the loading resistor value, logic code for selecting an amplifier mode based on the device impedance, logic code for calibrating a device gain and an amplifier gain, logic code for measuring noise data under the bias condition, logic code for removing an undesired portion of the noise data to obtain device noise data, logic code for extracting a model based on the device noise data, and logic code for simulating the extracted model.
In one embodiment, the logic code for calculating a supply voltage includes logic code for calculating an initial voltage value, logic code for measuring a supply current, logic code for comparing the supply current to a sum of a leak current and a device current, logic code for setting the supply voltage to the initial voltage if the supply current is equal to the sum, logic code for incrementing the supply voltage by a change voltage if the supply current is not equal to the sum and repeating the measuring and comparing. In another embodiment, the logic code for calculating a supply voltage further comprises logic code for receiving an error percentage and logic code for calculating the change voltage based on the error percentage.
In yet another embodiment, the logic code for selecting an amplifier mode includes logic code for measuring noise densities at a plurality of loading resistance and logic code for selecting a voltage amplifier mode or a current amplifier mode based on the measuring. In another embodiment, a voltage amplifier mode is selected when the device impedance is low and a current amplifier mode is selected when the device impedance is high.
In an exemplary embodiment, the logic code for calibrating a device gain and an amplifier gain includes logic code for providing an alternate current signal at an input terminal of the device, logic code for measuring an output voltage at an output terminal of the device, and logic code for calculating the device gain and the amplifier gain based on the input voltage and the output voltage. In another exemplary embodiment, the logic code for removing an undesired portion of the noise data includes logic code for determining a system noise and logic code for separating the system noise from the noise data.
In an exemplary embodiment, the logic code for extracting a model includes logic code for extracting a direct current model having direct current model parameters, logic code for extracting frequency dependent model parameters, logic code for extracting noise data at one or more sampling frequency, logic code for associating the noise data to at least one bias condition, and logic code for extracting a noise model based on the direct current model parameters and the association between the noise data to the at least one bias condition. In one embodiment, the logic code for simulating the model includes logic code for generating a test circuit based on the model, and logic code for simulating the test circuit.
An exemplary system for measuring noise in a device comprises a CPU, a memory coupled to the CPU, an interface coupled to the CPU for providing instructions processed by the CPU, a control unit coupled to the interface for receiving the instructions, a preamplifier circuit coupled to the control unit for implementing the instructions, a power supply unit controlled by the control unit for providing power to the preamplifier circuit, and a device holder selectively attached to the preamplifier circuit. In an exemplary embodiment, the control unit further comprises a front panel having a plurality of control buttons. the control buttons are configured to allow manual control of the preamplifier circuit and the power supply unit. In another embodiment, control unit further comprises a microprocessor for implementing a firmware when the control buttons are operated. In yet another embodiment, the preamplifier circuit further comprises a plurality of filters, an amplifier circuit, a plurality of switches for switching the amplifier circuit between a voltage amplifier mode and a current amplifier mode, and a variable loading resistor.